Ceramics for cutting tools

ABSTRACT

A ceramic material for cutting tools is provided. The ceramic material has a high thermal conductivity and an excellent resistance to chipping failure and comprises a total of 10 to 40% by weight of titanium boride and titanium carbide, and 0.5 to 10% by weight of zirconia, the balance being alumina and incidental impurities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ceramic materials, particularly suitable foruse as cutting tools.

2. Description of the Prior Art

Cutting Tools made of alumina, well known as one of the ceramics forcutting tools, are excellent in abrasion resistance, but, on the otherhand, have a disadvantage of being susceptible to chipping. For thisreason, the impovement in toughness has been one of the main objectivesof developmental work on ceramic tools from the early stage of theirdevelopment until now. As one of solutions to the problem, there wasinvented a ceramic tool of an alumina-titanium carbide (Al₂ O₃ -TiC)type with an improved resistance to the chipping caused by thermalshock, in which alumina was incorporated with about 30% by weight oftitanium carbide to reduce the size of crystal grains as well as toreduce the decline in thermal conductivity with the increase intemperature said decline originating from the inherent properties ofalumina.

As compared with a so-called "white-type" ceramic tool made of aluminaalone, the ceraminc tool of the Al₂ O₃ -TiC type has a higher toughnessand a higher resistance to the chipping failure caused by thermal shock.As a consequence, this type of tool has exhibited its usefulnessespecially in such uses as turning of high-hardness materials andmilling of cast iron materials, both of which are hardly machinable bymeans of "white" ceramic tools. However, even such a ceramic tool of anAl₂ O₃ -TiC type has a disadvantage of being susceptible to boundaryabrasion in the area of machining a high-hardness material under suchmachining conditions that the temperature of cutting edges exceeds1,200° C., because the high-temperature hardness of TiC incorporated ina considerable amount becomes lower than that of alumina, resulting indeterioration of the abrasion resistance of the tool.

SUMMARY OF THE INVENTION

The object of this invention is to provide a ceramic material forcutting tools, which has a high thermal conductivity and an excellentresistance to chipping failure, thus eliminating the disadvantages ofthe prior art.

The present invention is characterized by the incorporation of 0.1 to10% by weight of ZrO₂ in a mixture comprising powdered alumina and atotal of 10 to 40% by weight of TiB₂ and TiC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effect of ZrO₂ upon the sinterability.

FIG. 2 is a graph showing the toughness of cutting tools at hightemperatures.

FIG. 3 is a graph showing the toughness at normal temperatures.

FIG. 4 is a graph showing the relationship between the hardness and theTiB₂ /TiC ratio.

FIG. 5 is a graph showing the effectiveness of ZrO₂ to improve thetoughness.

FIG. 6 is a characteristics graph showing the results of cutting testperformed on each of the ceramics of this invention and a conventionalceramics of the Al₂ O₃ -TiC type.

FIGS. 7a and 7b are, respectively, graphs of the results of testperformed on the ceramics of this invention and a conventional ceramicsof the Al₂ O₃ -TiC type by using a milling cutter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The ceramic material of this invention has a microstructure in whichTiB₂, TiC and ZrO₂ are dispersed in Al₂ O₃ similarly to TiC in ceramicsof the Al₂ O₃ -TiC type and, as known well, play roles of preventing thegrowth of alumina grains to increase the toughness as well as hinderingthe growth of tiny cracks. However, when hardly sinterable TiB₂ is addedin a large amount to a sintered Al₂ O₃ -TiC mixture, there occurs anincrease in the firing temperature, resulting in an undesirable growthof the grain size to around 10 μm. The addition of ZrO₂ according tothis invention eliminates such a difficulty and is effective inimproving the sinterability of Al₂ O₃ -TiB₂ -TiC ceramics. For instance,as seen from FIG. 1, when ZrO₂ is added, the hot-pressing temperaturenecessary to achieve a density ratio of 99.5% or more, is at least 100°C. lower than that in the case where no ZrO₂ is added. Moreover,according to this invention it is possible to improve further thesinterability and to minimize the growth of grains by the addition of atotal of 0.05 to 2.5% by weight of a sintering aid such as magnesia(MgO) and aluminum nitride (AlN).

The ceramics of this invention are obtained normally by the hot pressingof a powder mixture having the composition in the range given above. Thesame result is attained by hot pressing followed by hydrostatic pressingwhile heating, or alternatively by firing under ordinary pressurefollowed by hot pressing under hydrostatic pressure.

The reason for limiting the total amount of TiB₂ and TiC to the range offrom 10 to 40% by weight in this invention is such that if the amount isbelow 10%, the resulting ceramic becomes inferior in toughness at hightemperatures, while if it exceeds 40%, the toughness at ordinarytemperature becomes inferior. For instance, the results shown in FIG. 2were obtained from the evaluation test of toughness at hightemperatures, which was carried out by machining a cast iron piece, usedas an example of the material to be milled, by means of a face millingcutter under such conditions that the milling speed was 500 m/minute andthe depth of cut was 1.5 mm. As is seen from FIG. 2, thehigh-temperature toughness of a ceramic tool containing less than 5% intotal or more than 50% in total of TiC and TiB₂ is inferior to that of aceramic tool containing 10 to 20% in total of TiC and TiB₂. As to thetoughness at ordinary temperature, test results obtained were shown inFIG. 3. As is seen from FIG. 3, when the total TiC and TiB₂ content isbelow 10% or above 40%, the toughness at ordinary temperature is rapidlydecreased. As is apparent from the results shown in FIGS. 2 and 3, it isdesirable that the total amount of TiC and TiB₂ be in the range of from10 to 40%. As is clear from FIG 4, it is more desirable to limit theweight ratio of TiB₂ to TiC to the range of from 0.15 to 6.0 in order toincrease markedly the hardness and, hence, to improve the abrasionresistance of the tool made from such a ceramic.

As for ZrO₂, a preferable amount is in the range of from 0.5 to 10%,because if the amount is below 0.5%, the ceramic composition hardlyexhibits any effect on sinterability as shown in the Table below, whileif the amount exceeds 10%, the resistance of the ceramics to thermalshock will be decreased.

    ______________________________________                                                       Optimum hot-pressing                                           Composition    temperature (°C.)                                       ______________________________________                                        Al.sub.2 O.sub.3 --12 TiC--                                                                  1650                                                           12 TiB.sub.2 --0.1 ZrO.sub.2                                                  Al.sub.2 O.sub.3 --12 TiC--                                                                  1650                                                           12 TiB.sub.2 --0.3 ZrO.sub.2                                                  Al.sub.2 O.sub.3 --12 TiC--                                                                  1580                                                           12 TiB.sub.2 --0.5 ZrO.sub.2                                                  Al.sub.2 O.sub.3 --12 TiC--                                                                  1500                                                           12 TiB.sub.2 --2 ZrO.sub.2                                                    Al.sub.2 O.sub.3 --12 TiC--                                                                  1450                                                           12 TiB.sub.2 --5 ZrO.sub.2                                                    ______________________________________                                    

The total amount of powdered MgO and AlN is limited to the range of from0.05 to 2.5% by weight, because if the total amount is below 0.05% byweight, no effect on the reduction in the crystal grain size of Al₂ O₃is seen, while if it exceeds 2.5% by weight, a spinel layer is formed atgrain boundaries and embrittles the ceramics.

The purpose of limiting the amount of Y₂ O₃ to the range of from 1.0 to3.0 mol % based on ZrO₂ is to make ZrO₂ in the crystal form oftetragonal (t phase) for the most part, thereby to improve the breakingstrength of the ceramics, as shown in FIG. 5, because if the amount ofY₂ O₃ is below 1.0%, the crystal form of ZrO₂ becomes mainly monoclinic(m phase), while if it is above 3.0%, most of the ZrO₂ crystals becomecubic (c phase), the effect of Y₂ O₃ being insufficient in these twocases.

The invention will be illustrated below with reference to the followingExamples.

EXAMPLE 1

To a mixture comprising 10% by weight of powdered TiB₂ of 1.2 μm in anaverage particle size, 10% by weight of TiC of 0.9 μm in an averageparticle size, 5% by weight of powdered ZrO₂ of 0.1 μm in an averageparticle size, and the balance of powdered Al₂ O₃, 0.1 μm in an averageparticle size, were added 0.5% by weight of MgO and 1.0% by weight ofAlN. The resulting mixture was ground and intermixed for 48 hours inethyl alcohol in a ball mill made of alumina. The ground mixture wasdried in vacuum, molded under a pressure of 2 tons/cm², and subjected topressure sintering keeping the molded piece at 1,500° C. in vacuum for10 minutes while applying a pressure of 200 kg/cm² to obtain a sinteredbody.

SNGN 453 tip for cutting tools was prepared from the sintered bodyabove. Cutting test was performed, using cold working steel (Hs 65) as apiece to be machined and an Al₂ O₃ -30% TiC tip, which has been widelyused, as a reference. The cutting speed was 70 m/minute, the depth ofcut 1.5 mm for both the tips, and the feed 0.4 mm/rev. As compared withthe conventional Al₂ O₃ -TiC ceramics, the ceramics of this inventionshowed lower boundary abrasion, indicating superior cutting performanceas shown in FIG. 6.

EXAMPLE 2

A mixture was prepared by blending 15% by weight of powdered TiB₂, 10%by weight of powdered TiC, 8% by weight of powdered ZrO₂, 0.29% byweight of powdered Y₂ O₃, and the balance of powdered Al₂ O₃. Themixture was ground and intermixed in water in a ball mill made ofalumina for 48 hours. The ground mixture was dried in vacuum, moldedunder a pressure of 2 tons/cm², and subjected to pressure sintering bykeeping the molded piece at 1,500° C. in vacuum under a pressure of 200kg/cm² to obtain a sintered body.

A test piece, 8×4×25 mm, was cut out of the sintered body above andtested for bending strength. The average bending strength was 85 kg/mm²which proved that the ceramics of this invention has a far higherstrength than that of a commercial Al² O₃ -30% TiC ceramics.

SNGN 432 tip for cutting tools was prepared from the above-mentionedsintered body. Cutting test was performed by means of a face millingcutter, using SCM 440 as a material to be machined and a commercial Al₂O₃ -30% TiC tip as a reference. The depth of cut was 2 mm for both thetips. As summarized in FIGS. 7a and 7b, the ceramics of this inventionshowed a broader machinable region than that of the commercial Al₂ O₃-TiC ceramics.

As is apparent from the foregoing Examples, the ceramics for cuttingtools according to this invention are very useful from industrialviewpoint, because they show excellent characteristics as the ceramicsfor so-called "black-type" tools, as evidenced by a low boundaryabrasion and a high toughness.

What is claimed is:
 1. A ceramic material for cutting tools comprising10 to 40% by weight in total of titanium boride (TiB₂) and titaniumcarbide (TiC) with the weight ratio of titanium boride to the titaniumcarbide being in the range of from 0.15 to 6.0, and 0.5 to 10% by weightof zirconia (ZrO₂), the balance being alumina (Al₂ O₃) and incidentalimpurities.
 2. The ceramic material for cutting tools according to claim1, further comprising at least one of aluminum nitride (AlN) andmagnesia (MgO) as sintering aid in a total amount of 0.05 to 2.5% byweight.
 3. A ceramic material for cutting tools comprising 10 to 40% byweight in total of titanium boride (TiB) and titanium carbide (TiC) withthe weight ratio of the titanium boride to the titanium carbide beingwithin the range of from 0.15 to 6.0, 0.5 to 10% by weight of zirconia(ZrO₂), and 1.0 to 3.0 mole %, based on the content of zirconia, ofyttria (Y₂ O₃), the balance being alumina (Al₂ O₃) and incidentalimpurities.
 4. The ceramic material for cutting tools according to claim3, further comprising at least one of aluminum nitride (AlN) andmagnesia (MgO) as sintering aid in a total amount of 0.05 to 2.5% byweight.